A typical communication network switch has a switching core and an array of communication network interfaces connected to the switching core. The network interfaces pass data traffic from the communication network to the switching core which directs the data traffic back to the appropriate communication interfaces for continued transmission over the network. Multiple network interface cards are accommodated in equipment shelves and are arranged to aggregate network traffic and feed the aggregated traffic to the central switching core. The capacity of a switching core has a maximum upper limit and therefore a service provider must select a switching core with the required capacity for the particular network. Since the demand for network bandwidth is not static, but typically increases over time, a service provider may either change the switching core from time to time to meet the required capacity, or purchase a switching whose capacity exceeds that of the present bandwidth requirements, with a view to connecting additional network interface cards to the switching core, as the demand for extra capacity increases. One drawback of the first option is that each time a switching core needs to be upgraded to a higher capacity switching core, network traffic cannot be serviced by the switch and therefore a second central core (providing redundancy to the active core) must be invoked, in which case redundancy is lost, or traffic must be re-routed around the switch, if possible. Customers may also experience a complete loss of service during the period over which the switching core is being replaced, and the service provider may suffer a loss of revenue. A drawback of the second option is that a service provider must invest in a switching core, part of whose potential capacity is not initially used, and therefore does not generate revenue, and may not generate any revenue for some time. High capacity switches, for example having a traffic handling capacity of the order of terabits are complex and expensive, and the cost of the initial investment in capacity to meet future demand and the period over which the capacity is underused may render these switches inaccessible to at least some service providers.
Communication network switches require a level of fault tolerance, and this is normally provided by duplicating the central switching core so that a second core can take over switching functions of the active core if the active core experiences a fault. The standby core usually has the same capacity as the active core and therefore the provision of a second switching core to provide fault tolerance significantly increases the cost, particularly for high capacity switches.